1. Field of the Invention
The present invention relates to an image forming apparatus applied to copying machines, facsimile apparatuses, printers and the like.
2. Prior Art
Conventionally, there has been known an image forming apparatus of the electrophotographic type for forming an electrostatic latent image on a photoreceptor, obtaining a toner image by developing with toner, and transferring the toner image on a sheet directly or through an intermediate transfer medium for fixing. The toner used for this electrophotographic type is solid particle-shaped toner prepared by adhering pigment into a styrene acryl or polyester binder with a diameter of about 7 to 10 .mu.M. as processes relating to toner movement, there are two known processes: developing and transfer processes, and variations in the positions of a group of toner particles such as toner scattering, pre-transfer and toner explosion pose a serious problem as image defects in both processes. By reducing a gap between the photoreceptor and the development sleeve as far as possible, actually bringing the developing electrode closer by using conductive toner to reduce toner scattering, controlling the posture of a sheet entering a transfer nip, removing the electric charge carried by a visible image formed on the photoreceptor, or de-electrifying the toner image transferred onto a recording medium, the above-described problem can be considerably reduced.
When, however, the toner particle diameter is further rendered minute to improve the image quality, the electric field condition in which the toner is handled is completely changed. When the toner particle diameter is reduced to half from 10 .mu.m to 5 .mu.m, the amount of charge is reduced to 1/4 if the toner charging condition is constant because the amount of charge which simple toner is capable of charging is in proportion to its surface area. Namely, the force F to which the toner is subjected from an electrostatic field E is reduced to 1/4 as shown below, and the effective driving force exerting on the particle reduces with the square of the reduction rate of the diameter EQU F=q'.multidot.E,S=4.pi.r.sup.2
where S: Surface area of particle, r: Particle radius, q: Amount of charge on toner, and q': Amount of charge on toner (particle diameter1/2), EQU q'=f(S).fwdarw.q'=1/4.times.q.multidot.E .thrfore.F=q'.multidot.E=1/4.times.q
On the other hand, as an adhesive force between a particle and a member for supporting and arranging the particle, a van der Waals force Fv is always acting in addition to an electrostatic force, and this van der Waals force Fv is in proportion to the radius r as represented by the following equation: EQU Fv={hw/(8.pi.r.sup.2)}i .times.r
where hw: Lifshitz-van der Waals force constant, z: object-to-object distance, r: particle radius. This means that when the toner particle diameter is rendered minute to improve the image quality, the van der Waals force required for rendering the diameter minute can be reduced by an amount corresponding to the ratio in diameter, but the amount of charge which can be charged for the square of the reduction ratio in diameter is reduced conversely. Therefore, the adhesive force Fv due to the van der Waals force becomes predominant even if an electric field greater than the maximum electric field (spark discharge starting electric field) based on the Paschen Rule may be imparted, and the electric-field tolerance in which transfer can be performed while toner is being sufficiently controlled will become exceedingly narrow. Also, in the case of only one layer of the toner particles, the van der Waals force Fv becomes extraordinarily greater than an adhesive force acting between the toner particles in the case of a multi-layer toner having a plurality of toner particles stacked. Therefore, relationship between the van der Waals force Fv and driving forces for movement, flying, transfer and adhesion which are capable of effectively acting on the particles becomes important, and in the conventional electrophotographic type, it is difficult to move the toner by imparting an electrostatic field within a range not exceeding the Paschen Rule to the one-layer toner.
In the developing and transfer processes using conventional multi-layer toner, the toner scattering occurs as described above. Since the adhesive force between the toner particles is weak, spatial extent of the electric field or the magnetic field causes transfer onto undesirable pixel positions conversely, or drop or scattering occurs because of multi-layer even if adhered to desirable pixel positions. Also, in order to secure developing density or transfer density higher than a certain level, an excessive electric field or magnetic field is imparted, and there may be also seen a phenomenon in which toner particles bounce and are scattered around even if they are shot onto desired pixel positions. For example, FIG. 1 shows a principle diagram for Toner Jet Type of Array Printers AB Inc., and a tandem type color image sample was presented for the first time in the NIP12 IS&T 1996, which has image quality of very conspicuous toner scattering. This type adopts the directly-recording technique using no photoreceptor, and there are provided mesh electrodes 5 between a development sleeve 1 and a sheet 3, and a back plate 2 on the back of the sheet, a voltage 8 of 1500 V is applied between the development sleeve 1 and the back plate 2, and an electric field based on control voltage 7 of 275 V is imparted to the mesh electrode 5 when an image is formed (toner passes through the mesh holes) and an electric field based on control voltage 7 of -50 V is imparted when no image is formed (toner does not pass through the mesh holes) to form an image. This type is an image forming method for controlling the toner as a mass under the electric field imparted and not controlling the simple toner individually. Therefore, the technical problems are toner clogging into the mesh electrode holes, adhesion or contamination to the mesh electrodes themselves, and toner scattering 10 caused by bounce of the toner shot onto the sheet 3 as shown in FIG. 2, resulting in problems such as necessity of cleaning and worsened graininess. Also, when there are variations in speed of the sheet 3 conveyed, no effects are exhibited to the variations in the speed by the control using the mesh electrode.
In Japanese Published Unexamined Patent Application No. Hei 3-240072, there is described the recording technique for forming an image at low potential gradient using such mesh electrode and a reference electrode for imparting a DC electric field. As problems associated with this type, the problems of clogging of the mesh electrode holes with toner and contamination by adhesion to the mesh electrode itself have been pointed out frequently, and in addition, the magnitude of the adhesive force on causing the toner to fly from the development sleeve has been pointed out. When the adhesive force based on the van der Waals force between the toner and the development sleeve is strong, the toner cannot fly at low electric potential, but it becomes necessary to impart considerably high voltage. Therefore, the risk factor of breaking down becomes higher in view of waste energy, environment and change with time.
Oce' 3125C Euro Color Copier is 400 dpi ring electrodes arranged in the axial direction of the imaging drum, and voltage based on the image information is imparted to driving electrodes provided within the imaging drum to develop the toner from a magnet roll. In this type, conductive magnetic mono-component toner is used, and an image formed on the imaging drum is of a single layer. Therefore, in the case of forming a full color image, it is necessary to rotate the imaging drum and the intermediate member drum with a considerably high degree of accuracy or to improve the rigidity of the constituent members in advance, leading to higher costs.
In the case of a type of recording by moving toner in this way, there is certainly a gap between the development sleeve and a sheet, and variations in the speed between them cause deviations in pixel positions, resulting in image defects such as color deviation and banding, or problems associated with reliability such as contamination and clogging caused by toner adhesion on the control electrodes. Further, the most serious problem of the prior art is poor peeling property between the development sleeve and the toner, and various countermeasures against this are considered.
Problems to Be Solved by the Invention
The present invention has been achieved in the light of the above-described state of affairs, and is aimed to provide an image forming apparatus capable of properly controlling individual positions of particles for image formation to thereby form a high quality image.